1. Field of the Invention
The present invention relates to the spin-injection write type magnetic memory device; for example, to the structure of a memory cell array.
2. Description of the Related Art
A magnetic random access memory (MRAM) uses a magnetoresistance effect element as a memory cell. The magnetoresistance effect element includes a free layer (recording layer) having a free magnetization (spin) direction, and a pinned layer having a fixed magnetization direction. The magnetoresistance effect element further includes a nonmagnetic layer, which is interposed between the two layers. When the magnetization direction of the free layer is parallel with that of the pinned layer, the element exhibits the low resistance. On the other hand, when the magnetization direction of the free layer is antiparallel with that of the pinned layer, the element exhibits the higher resistance. The difference of the resistance is used to record information.
To read information, a read current is carried into the magnetoresistance effect element and a current or voltage value which depends on the resistance state of the magnetoresistance effect element is compared with a reference value to determine the resistance state. To write information, current is carried through two write lines which are orthogonally cross at a memory cell to generate a magnetic field and the magnetic field switches the magnetization of the free layer. The memory cells are arrayed to configure a memory cell array.
With the scale-down of the magnetic memory device, the distance between components such as write lines and magnetoresistance effect elements is narrowed. For this reason, in the magnetic memory device using the magnetic field for writing, the magnetic field from the write line carrying a write current is highly possible to unintentionally write data in a memory cell which lies near the write line and is not a write target. This tendency more remarkably appears with the scale-down.
Moreover, the magnetic field write method requires a large current to generate a magnetic filed large enough for writing. This makes it hard to reduce power consumption.
On the contrary, spin-injection write (magnetization switching) has attracted interests (see U.S. Pat. No. 5,695,864). This write method involves carrying a current which is spin-polarized by a magnetic moment of a pinned layer to a free layer of a magnetoresistance effect element to change its magnetization direction to write data in accordance with the current direction. According to the method, a direct action is given to nano-scale magnetic materials as compared with the magnetic field write. Therefore, unintentional write into neighboring memory cells can be avoided, and high-speed magnetization switching can be expected. Moreover, using this write method, the write current is reduced by scaling down the memory cell.
The spin-injection write method requires bi-directional write current carried into the magnetoresistance effect element in accordance with written information. Due to this requirement, attention must be given to avoid the unintentional write into unselected memory cells and write speed decrease resulting from charge/discharge of the parasitic capacity of the write line. Moreover, the spin-injection write method does not require the magnetoresistance effect element to be held between two write lines, unlike the magnetic field write method. Therefore, the connection and the positional relationship between memory cells and interconnects used in the magnetic filed write method should not be applicable to the spin-injection write method.
Arrangement of memory cells and interconnects which can realize the spin-injection write method is insufficient. In addition, high integration needs to be realized.